In vivo translation of the CIRPI system: Revealing molecular pathology of rabbit aortic atherosclerotic plaques

Thin-cap fibroatheroma (TCFA) are the unstable lesions in coronary artery disease that are prone to rupture, resulting in substantial morbidity and mortality worldwide. However, their small size and complex morphologic and biologic features make early detection and risk assessment difficult. We tested our newly developed catheter-based Circumferential-Intravascular-Radioluminescence-Photoacoustic-Imaging (CIRPI) system in vivo to enable detection and characterization of vulnerable plaque structure and biology in rabbit abdominal aorta. Methods: The CIRPI system includes a novel optical probe combining circumferential radioluminescence imaging and photoacoustic tomography (PAT). The probe's CaF₂: Eu-based scintillating imaging window captures radioluminescence images (360° view) of plaques by detecting β-particles during ¹⁸F-FDG decay. A tunable laser-based PAT characterizes tissue constituents of plaque at 7 different wavelengths-540 and 560 nm (calcification), 920 nm (cholesteryl ester), 1040 nm (phospholipids), 1180 nm (elastin/collagen), 1210 nm (cholesterol), and 1235 nm (triglyceride). A single B-scan is concatenated from 330 A-lines captured during a 360° rotation. The abdominal aorta was imaged in vivo in both atherosclerotic rabbits (Watanabe Heritable Hyper Lipidemic [WHHL], 13-mo-old male, n 5 5) and controls (New Zealand White, n 5 2). Rabbits were fasted for 6 h before 5.55 · 10⁷ Bq (1.5 mCi) of ¹⁸F-FDG were injected 1 h before the imaging procedure. Rabbits were anesthetized, and the right or left common carotid artery was surgically exposed. An 8 French catheter sheath was inserted into the common carotid artery, and a 0.035-cm (0.014-in) guidewire was advanced to the iliac artery, guided by x-ray fluoroscopy. A bare metal stent was implanted in the dorsal abdominal aorta as a landmark, followed by the 7 French imaging catheters that were advanced up to the proximal stent edge. Our CIRPI and clinical optical coherence tomography (OCT) were performed using pullback and nonocclusive flushing techniques. After imaging with the CIRPI system, the descending aorta was flushed with contrast agent, and OCT images were obtained with a pullback speed of 20 mm/s, providing images at 100 frames/s. Results were verified with histochemical analysis. Results: Our CIRPI system successfully detected the locations and characterized both stable and vulnerable aortic plaques in vivo among all WHHL rabbits. Calcification was detected from the stable plaque (540 and 560 nm), whereas TCFA exhibited phospholipids/cholesterol (1040 nm, 1210 nm). These findings were further verified with the clinical OCT system showing an area of low attenuation filled with lipids within TCFA. PAT images illustrated broken elastic fiber/collagen that could be verified with the histochemical analysis. All WHHL rabbits exhibited sparse to severe macrophages. Only 4 rabbits showed both moderate-to-severe level of calcifications and cholesterol clefts. However, all rabbits exhibited broken elastic fibers and collagen deposition. Control rabbits showed normal wall thickness with no presence of plaque tissue compositions. These findings were verified with OCT and histochemical analysis. Conclusion: Our novel multimodality hybrid system has been successfully translated to in vivo evaluation of atherosclerotic plaque structure and biology in a preclinical rabbit model. This system proposed a paradigm shift that unites molecular and pathologic imaging technologies. Therefore, the system may enhance the clinical evaluation of TCFA, as well as expand our understanding of coronary artery disease.